Conclusions

A 4-year study, which included field measurement of ECa, identified the effects of manure, compost, fertilizer, and cover crop on ECa values. Compost and manure applied at the N rate resulted in consistently higher conductivity and available N, when compared to the commercial fertilizer and

manure and compost at the phosphorus rate (which had not been applied since 1998). Sequential measurements of profile weighted ECa effectively identified the dynamic changes in available soil N, as affected by animal manure and N fertilizer treatments, during the corn growing season. The sequential measurements also clearly identified the effectiveness of cover crops in minimizing levels of available soil N before and after the corn-growing season, when soluble N is most subject to loss. Ferguson et al. (2003) reported that use of a winter cover crop was effective in reducing NO3-N levels at depths below 0.5 m. Nitrate levels from 0.5 m to the surface tended to be higher under the cover crop, indicating that the cover crop is releasing NO3-N in the upper portion of the soil profile. Sequential ECa graphs indicate that the immobilized NO3-N is released when the +CC and -CC curves converge. This 4-year study supports the initial findings of a 1999 study that soil conductivity appears to be a reliable indicator of soluble N gains and losses in the soil under study, and may serve as a measure of N sufficiency for corn early in the growing season, as well as an indicator of N surplus after harvest when N is prone to loss from leaching and runoff.